This invention relates to free-piston Stirling engines, coolers and heat pumps and more particularly relates to a position sensor for sensing the position of the reciprocating power piston of such a free-piston machine. The sensor has an advantage that it adds only insignificantly to the length of the free-piston machine. Also it has the advantage that the coil part of the sensor, which provides the electrical output signal, can be mounted to the exterior of the pressure vessel head of the free-piston machine so that it does not require that any electrical conductors be fed through the pressure vessel.
Provisional patent application Ser. No. 61/305,986, filed Feb. 19, 2010 and from which priority is claimed, is hereby incorporated in this application by reference. U.S. Pat. Nos. 4,667,158; 4,866,378; and 4,912,409 are also hereby incorporated by reference.
Free-piston Stirling machines are thermo-mechanical oscillators that are well known in the prior art. Free piston Stirling machines offer numerous advantages including the ability to control their frequency, phase and amplitude, the ability to be hermetically sealed from their surroundings and their lack of a requirement for a mechanical fluid seal between moving parts to prevent the mixing of the working gas and lubricating oil. Generally, a free-piston Stirling machine includes a power piston that reciprocates in a cylinder and is attached to a spring to form a resonant system. The power piston is also attached to a load, when the Stirling machine is operated as an engine, and to a reciprocating prime mover for driving the piston in reciprocation when the Stirling machine is operated as a heat pump or cooler. In a free-piston Stirling machine the piston, and a displacer if there is one, are not connected to each other or to a load or prime mover by a mechanical linkage that confines their reciprocation to a fixed stroke, for example connecting rods and crankshafts. Instead, the stroke of the oscillating piston is free to vary.
Free-piston machines are typically designed with the piston having a nominal design stroke. However, as the machine encounters varying operating parameters, such as varying loads or varying operating temperatures, the piston stroke varies away from the nominal design stroke since its stroke is not confined by mechanical linkages. If the operating stroke increases sufficiently, the piston can collide with other mechanical structures of the machine such as a displacer or parts fixed at axially opposite ends of the cylinder in which the piston reciprocates.
Because of the variations in stroke as a function of operating parameters and the possibility of collisions, free-piston machines commonly have an electronic control system. One of the most important parameters that are sensed and used by the control system is the linear position of the piston. For example, sometimes it is desirable to sense the instantaneous linear position or translation of the piston as it reciprocates through its cyclic motion and/or to sense the opposite end limits of the piston reciprocation.
The above three cited U.S. patents to Robert W. Redlich show a position sensor that has an elongated coil and a tube that reciprocates in and out of the coil. The inductance and therefore the impedance of the coil decreases as a function of the length of insertion of the tube into the coil. Although the Redlich sensor is effective, it occupies a length within the Stirling machine that is on the order of at least twice the stroke of the piston. The reason is that, in order to sense the position along the entire stroke, both the coil and the tube must have a length at least equal to the stroke. The tube must be able to reciprocate between a position of maximum withdrawal from the coil to a position of maximum insertion into the coil and the distance between maximum withdrawal and maximum insertion must be at least equal to the stroke. Consequently, the length of the entire Redlich sensor must be at least twice the length of the stroke at the position of maximum withdrawal. In addition to the length of the Redlich sensor, the design of the Stirling machine must provide for locating both the coil and the reciprocating tube within the machine. Consequently, the Redlich sensor adds volume and length requirements to a free-piston Stirling machine. Furthermore, because the coil of the Redlich sensor must be located within the hermetically sealed pressure vessel of the Stirling machine, the electrical conductor leads from the coil must extend through the pressure vessel wall for connection to control circuitry. That reduces the reliability of the machine because such electrical feedthroughs must be sealed to withstand a high pressure. The seals provide an additional failure risk.
It is an object and feature of the present invention to provide a position sensor that adds only negligibly to the length and volume of a free-piston Stirling machine.
Another object and feature of the invention is to provide an embodiment of the invention that requires no electrical conductor leads extending through the pressure vessel.